Musnad Abu Ya'la al-Mawsili 3:226

Suwayd narrated to us, Salih bin Musa bin Ishaq bin Talha al-Qurashi narrated to us, from Abdullah bin al-Hasan, from his mother Fatima bint al-Husayn, from her father, from Ali, that the Messenger of Allah (may Allah bless him and grant him peace) used to say when he entered the mosque: "O Allah, open for me the doors of Your mercy." And when he exited, he would say: "O Allah, open for me the doors of Your bounty."